Many integrated circuit die (ICs) are manufactured on a single substrate. An IC is constructed by patterning each layer grown on the substrate. Commercially, a stepper exposes each layer with many images. Each new image must be precisely positioned over the image on the previous layer to provide a functioning IC. A laser interferometer measurement system that can resolve distances with a resolution on the order of 1 nanometer is used as part of the precision positioning system. A typical measurement system includes a source of laser light, an optical interferometer, a reflector, a receiver, and some electronics. The receiver outputs a measurement signal that changes phase as the reflector is moved with respect to the interferometer. The laser light source provides a reference signal having a constant frequency. The electronics compare the measurement signal with the reference signal to determine the distance moved by the reflector.
The phase difference between the reference and measurement signals is directly proportional to the change in position of the reflector with respect to the interferometer. The main purpose of the electronics is to accumulate this change in phase and convert it into units of position. As the reflector moves back and forth, the phase of the receiver signal increases and decreases with respect to the reference phase. The electronics must keep up with the phase of these signals, continuously incrementing or decrementing a counter to keep track of the position of the reflector.
The reference and measurement signals are sinusoidal. The electronics determine the phase of these by precisely measuring the time of their zero crossings. When phase is determined from zero crossings, it is important that when either signal crosses zero, that it is only counted once. With large laser measurement systems, the laser light from one laser source is often split into many beams to supply multiple axes. As the number of beams that split from one source increases, the intensity of the light that reaches the receivers will decrease. The electrical noise in the receiver adds to the signal from the incoming light. When a weak measurement signal crosses zero, the electrical noise may dominate and may cause detection of a "false" zero crossing (also known as a glitch). If there is just one extra zero crossing, this causes an extra 360.degree.of phase to be accumulated by the electronics and hence an error occurs. Since the light intensity of the laser source is limited, the number of times that the light can be split from the source depends upon the sensitivity of the receiver. With complex steppers, ill is desirable to split the light beam as many times as possible. Therefore, an improvement in the sensitivity of the receiver reduces the cost of the stepper.
The phase of the measurement signal increases or decreases when the reflector position moves forward or backward. As a result, the frequency of the measurement signal changes with the velocity of the reflector. The frequency will doppler shift up for a velocity in one direction and down for a velocity in the other direction. As IC production requirements continue to increase, the demands on steppers have also increased. Large wafer runs require steppers with higher wafer throughput. Laser interferometers used in such systems must then be able to operate over a wider frequency range since the steppers must operate at higher velocities. A receiver that can operate over a wider frequency range must have a wider bandwidth. This wider bandwidth receiver will exhibit more noise, further complicating the zero crossing detection problem.
A laser interferometer measurement system that reduces detection of "false" zero crossings would allow laser signals of smaller intensity to be used. This would allow more light signals to be split off from the source and would reduce the cost of the resulting wafer stepper. It would be further beneficial if the measurement system were able to resolve differences in phase between the measurement and reference signals during higher stage velocities. This would result in more efficient positioning.